Vascular cells within the heart and blood vessels are subject to an array of signals which induces adaptive responses for the maintenance of homeostasis. Because cardiomyocytes are incapable of self-renewal, the heart possesses limited means of responding to biochemical, inflammatory, oxidative, or ischemic stresses. Annexin II is a calcium- regulated, phospholipid-binding endothelial cell surface protein that serves as a co-receptor for endothelial cell derived tissue plasminogen activator and its circulating substrate, plasminogen. Purified annexin II stimulates the catalytic efficiency of plasmin generation by approximately 60-fold. We have developed an annexin II-deficient mouse and observed that these animals develop a degenerative cardiomyopathy with impaired myocardial function. We have further observed that, in endothelial cells propagated in vitro, annexin II is exported to the cell surface via a non- classical secretion pathway in response to stress. The overall goal of this project is to understand the role of annexin II in cardiac function both at baseline and during states of cellular stress. We hypothesize that annexin II may play a critical cardioprotective role by maintaining the integrity of the myocardial microvascular circulation. Aim I will to determine the cardiovascular consequences of annexin II deficiency in mice. Using our recently developed annexin II knockout mouse, we will study the mechanism by which annexin II deficiency may lead to reduced myocardial contractility, abnormal ventricular architecture and cardiac arrthymia. Aim II will be able to determine the mechanism by which annexin II participates in the endothelial cell stress response. Preliminary data indicate that heat shock and other endothelial stresses lead to protein synthesis-independent, rapid release of annexin II to the cell surface with a concomitant increase in cell surface fibrinolytic activity. This export process does not require an intact endoplasmic reticulum-Golgi axis, and is inhibited by PD98059, , an agent that blocks activation of the Erk1/Erk2 MAP kinase signaling cascade. We plan to elucidate this export mechanism and to determine its effect on endothelial cell function. Aim III, finally, will e to determine whether annexin II is cardioprotective during cardiovascular stress states in vivo. Annexin II has been found to be up-regulated in the hearts of humans and animals with cardiac dysfunction. Using models of acute, subacute and chronic stress induced by adrenergic stimuli, exercise, heat stress, biome3chanical compromise, or oxidant agents, we will determine whether annexin II protects against cardiovascular compromise.